Morbidly A Beast:
I don’t really see the point unless you’re not actively gaining or fat, could be useful for feeders? It’s pretty obvious you can tell us by our bellies… but obviously not all fat people are in on our thing.
I've spoken to a number of feeders on FF. I've notice that the repressed (usually male) feeders have fetishized fatness so much that neutral behaviors become signals to them. Things like wearing ill-fitting clothes, weight gain (especially if the person doesn't make an effort to lose the weight), overeating, etc.
If we had a community-wide symbol, it would eliminate the need to ask someone if they are a feedist. However, I foresee this encouraging objectification and ostracization. Secret symbols don't stay secret for long. The mainstream crowd doesn't like us. Sooner or later, they'll find out about it and go on hate brigades.
Kink discussions are probably the safest option for now. This seems to be the case with most fetishes - even mainstream ones.